As the information super-highway continues its frantic expansion, an increasing portion of the computer and communications systems being deployed and in development utilize wireless technologies. One of the key signaling techniques being used in many of these systems is direct sequence code-division multiple access (DS-CDMA) which is one method of spread-spectrum. (See references [1]-[4], hereby incorporated by reference.) Monolithic implementations of receivers for spread spectrum systems typically implement much of the signal processing in the digital domain (See references [1]-[3]). Two important parameters of such implementations are chip cost and power consumption; power consumption being of particular importance in portable applications in order to prolong battery life. For low chip cost, minimum die size in digital CMOS technologies is desirable.
Many systems used in wireless technologies utilize a correlator for correlating the spread-spectrum data signal. One conventional approach for analog implementation of the correlator is to utilize Surface Acoustic Wave (SAW) filters. However, these are not amenable to integrated circuit implementation and are not economically attractive. Charge Coupled Devices (CCD) are another analog approach used to correlate signals. CCDs can be integrated into standard CMOS technology. However, CCDs: i) require additional steps of manufacturing, increasing overall cost; ii) require high-voltage clocking which results in increased power consumption; and iii) require high-voltages are undesirable in battery and portable applications. Another approach to the correlation function is what is referred to as Ad Hoc Mixed-Signal Implementations. These ad hoc approaches greatly compromise the signal processing by implementing gross approximations of the required signal processing, resulting in large performance losses. These ad hoc approaches are less attractive than digital implementations which perform better and can be more easily integrated. Another approach for analog implementation of the correlator is described in K. Onodera and P. Gray (See reference [5]), which uses Switched-Capacitor (SC) techniques. The incoming signal is sampled at twice the chip rate onto capacitors and the sampled voltages are subsequently summed in the charge domain. The SC technique described by K. Onodera and P. Gray underscore some low-power advantages of analog processing by placing the analog-to-digital converter after the correlation, allowing a lower-sampling rate and thus lower-power analog-to-digital converter. However, conventional SC sampling arrays require large areas on the chip die which translates to higher manufacturing cost.
By far the most widespread correlation technique used today is through digital implementation. Analog-to-digital (A/D) converters are used to convert an analog input signal to the digital domain where correlation and all other processing is performed. These techniques are effective for low bit rate systems. But as data rates increase, the sampling rate of the AID converter increases much faster. Thus, digital implementations for future higher rate systems will required an increased power consumption which is undesirable for battery operated portable systems. The performance of the digital correlators is usually compromised by the use of a smaller number of bits in the A/D conversion and processing which affects the size and power of the circuit implementation. The use of a smaller number of bits also compromises the attainable signal processing performance of DS-CDMA signaling. AID converters use smaller number of bits prior to the correlation of the signal that reduces the robustness to continuous-wave interferers. The small number of bits in the A/D conversion makes the system very sensitive to the input dynamic range, requiring complex transmitting power control mechanisms along with the need for a high performance gain control mechanism in the receivers. Other descriptions of spread spectrum techniques are available in the technical literature, including for example, a review article provided in R. Dixon, Spread Spectrum Systems with Commercial Applications, Third Edition, John Wiley & Sons, Inc., 1994, which is hereby incorporated by reference.